In regards to time and aging I have a line from Star Trek: Generations that comes back to mind from time to time. It came back to mind today reading these posts (and coming up on 52 in a couple weeks).
(Paraphrasing)
Picard: I realized there are fewer days ahead of me than behind me.
I think all the factors we have been posting are valid and equally important. I also think something like line fatigue is a big factor. By fatigue I mean a line gets too long in the tooth and starts to repeat itself and those of us who have been going along all these years can start to see there is little point to staying on the treadmill for minor upgrades to figures. Classified has been lucky in that Joe has not ever had a 6" line with classic ARAH designs. So it can feel a bit more fresh.
However lines like SW, ML and TF have been pumping out the same figures/characters in the same scales (roughly) for so long that they have repeated, and slightly upgraded, so many figures/characters that it really does feel like you are buying the same figure for a higher price at times. That lends itself to line fatigue.
I also think about how long some of these hobbies/brands can last. There will always be new (younger) fans for any hobby/brand, but eventually even the strongest brands fade to a much more niche interest. For something like action figures that is particularly interesting. Big companies like Hasbro and Mattel won't keep making stuff when the interest fades as evidenced by most brands (GI Joe, ML, SW) which have all taken a "hiatus" over the years. And that is during the golden age of collecting, so what happens when our generation ages out (dies) and, generously, the remaining fandom is halved?